Super Absorbent Polymer

ABSTRACT

The present invention relates to a super absorbent polymer. The super absorbent polymer according to the present invention includes surface crosslinked polymer particles prepared by surface crosslinking particles of a base resin, wherein the base resin is polymerized from water-soluble ethylene-based unsaturated monomers, and a water-soluble component, in which the water-soluble component is present in an amount of 5% by weight or less based on the total weight of the super absorbent polymer, when measured after free swelling the super absorbent polymer in a 0.9% NaCl solution for 1 hour, and the super absorbent polymer has a permeability of 50 seconds or shorter. According to the present invention, provided is a super absorbent polymer having excellent liquid permeability even when swollen without a reduction in centrifuge retention capacity or absorbency under load while having improved permeability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/882,619 filed Oct. 14, 2015, which claims the benefit of KoreanPatent Application No. 10-2015-0007918 filed on Jan. 16, 2015, thedisclosures of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a super absorbent polymer, and moreparticularly, to a super absorbent polymer having excellent liquidpermeability without a reduction in other physical properties.

(b) Description of the Related Art

A super absorbent polymer (SAP) is a type of synthetic polymericmaterials capable of absorbing moisture from 500 to 1000 times its ownweight. Various manufacturers have denominated it as different names,such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material),etc. Since such super absorbent polymers started to be practicallyapplied in sanitary products, now they have been widely used not onlyfor hygiene products such as disposable diapers for children, etc., butalso for water retaining soil products for gardening, water stopmaterials for the civil engineering and construction, sheets for raisingseedling, fresh-keeping agents for food distribution fields, materialsfor poultice or the like.

As a preparation process for such super absorbent polymers, a process bya reverse phase suspension polymerization and a process by a solutionpolymerization have been known. For example, Japanese Patent Laid-openPublication Nos. S56-161408, S57-158209, and S57-198714 disclose thereverse phase suspension polymerization. The process by the solutionpolymerization further includes a thermal polymerization method in whicha polymerization gel is polymerized while being broken and cooled in akneader equipped with a plurality of shafts, and a photo-polymerizationmethod in which an aqueous solution with a high concentration isirradiated with UV rays onto a belt to be polymerized and dried at thesame time. The hydrogel polymers thus obtained through thepolymerization reaction are generally marketed in a powdery form afterdrying and pulverization processes.

Meanwhile, in the preparation process of the super absorbent polymer, awater-soluble component which is an uncrosslinked polymer is produced. Ahigh content of the water-soluble component increases a liquidabsorption property of the super absorbent polymer. However, when thesuper absorbent polymer is in contact with a liquid, it is easily elutedto make the surface sticky or to cause an unfavorable effect when beingin contact with the skin. Further, if the content of the water-solublecomponent is high, the eluted water-soluble component generally existson the surface of the super absorbent polymer and it makes the superabsorbent polymer sticky, leading to a reduction in liquid permeabilitywhich is an ability to rapidly transfer a liquid to other superabsorbent polymer.

Accordingly, there is a demand to develop a super absorbent polymerhaving excellent liquid permeability while maintaining high waterabsorption property.

SUMMARY OF THE INVENTION

In order to solve the above conventional technical problems, an aspectof the present invention is to provide a super absorbent polymer havingexcellent liquid permeability even when swollen without a reduction incentrifuge retention capacity or absorbency under load.

In some aspects, the present invention provides a super absorbentpolymer including surface crosslinked polymer particles prepared bysurface crosslinking particles of a base resin, wherein the base resinis polymerized from water-soluble ethylene-based unsaturated monomers,and a water-soluble component,

wherein the water-soluble component is present in an amount of 5% byweight or less based on the total weight of the super absorbent polymer,when measured after free swelling the super absorbent polymer in a 0.9%NaCl solution for 1 hour, and

wherein the super absorbent polymer has a permeability calculated by thefollowing Equation 1 of about 50 seconds or shorter:

Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein T_(S) (sec) is the time required for an amount of a 0.9% salinesolution to permeate a saline-absorbed super absorbent polymer under aload of 0.3 psi, wherein the saline-absorbed super absorbent polymer isprepared by swelling 0.2 g of super absorbent polymer powder with the0.9% saline solution for 30 minutes, and

T₀ (sec) is the time required for the amount of the 0.9% saline solutionto flow under the load of 0.3 psi in the absence of the saline-absorbedsuper absorbent polymer.

According to the super absorbent polymer of the present invention,provided is a super absorbent polymer having excellent liquidpermeability even when swollen without a reduction in centrifugeretention capacity or absorbency under load while having improvedpermeability.

DETAILED DESCRIPTION OF THE EMBODIMENTS

While the present invention is susceptible to various modifications andalternative forms, specific embodiments will be illustrated anddescribed in detail as follows. It should be understood, however, thatthe description is not intended to limit the present invention to theparticular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

In addition, the term ‘include’ or ‘comprise’ means that it includes aparticular component (or particular element) without particularlimitations unless otherwise mentioned in the present entire disclosure,and it cannot be interpreted as it excludes the addition of the othercomponents.

As used herein, the term “particle size” refers to a sieve size used topartition the particles, unless otherwise specified.

As used herein, the terms “content distribution”, “molecular weightdistribution” or “ratio of dwt/d(log M)” refer to amounts of molecularweights present in a water-soluble component of super absorbent polymer,unless otherwise specified, and these terms can be used interchangeably.

Hereinafter, a super absorbent polymer according to an embodiment of thepresent invention will be described in detail.

The super absorbent polymer according to an embodiment of the presentinvention includes surface crosslinked polymer particles prepared bysurface crosslinking particles of a base resin, wherein the base resinis polymerized from water-soluble ethylene-based unsaturated monomers,and a water-soluble component, wherein the water-soluble component ispresent in an amount of 5% by weight or less based on the total weightof the super absorbent polymer, when measured after free swelling thesuper absorbent polymer in a 0.9% NaCl solution for 1 hour, and

wherein the super absorbent polymer has a permeability calculated by thefollowing Equation 1 of about 50 seconds or shorter:

Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein T_(S) (sec) is the time required for an amount of a 0.9% salinesolution to permeate a saline-absorbed super absorbent polymer under aload of 0.3 psi, wherein the saline-absorbed super absorbent polymer isprepared by swelling 0.2 g of super absorbent polymer powder with the0.9% saline solution for 30 minutes, and

T₀ (sec) is the time required for the amount of the 0.9% saline solutionto flow under the load of 0.3 psi in the absence of the saline-absorbedsuper absorbent polymer.

During a preparation process of the super absorbent polymer, awater-soluble component which is an uncrosslinked polymer is produced.The total content of the water-soluble component and the molecularweight of the water-soluble component greatly differ depending on thecontent of an initiator, polymerization temperature, content of acrosslinking agent, conditions for a surface crosslinking process, etc.during the polymerization.

A high content of the water-soluble component increases a waterabsorption property of the super absorbent polymer. However, when thesuper absorbent polymer is in contact with a liquid, the water-solublecomponent is easily eluted, and thus the surface of a diaper may becomesticky or the skin may be damaged. Meanwhile, if the content of thewater-soluble component is high, the eluted water-soluble componentgenerally remains on the surface of the super absorbent polymer andmakes the super absorbent polymer sticky, leading to a reduction inliquid permeability. Since the absorbency and the liquid permeabilityare conflicting properties, the super absorbent polymer may have veryexcellent physical properties by improving both of these properties. Inparticular, these properties are more important, for example,considering the current trend of slimness of the diaper thickness.

On the other hand, the water-soluble component in the super absorbentpolymer may exist in an uncrosslinked form due to an incompletecrosslinking reaction during the polymerization. However, most of thewater-soluble component may be generated by degradation of thecrosslinking agent or breakage of a main polymer chain during a dryingprocess. In this case, a free polymer chain of which one end iscrosslinked but the other end is uncrosslinked, rather than acrosslinked chain, is generated, and the free polymer chain is eluted asthe water-soluble component when the polymer chain is broken by heat.Such water-soluble component causes a problem of liquid permeability ordiscomfort in a swollen state by absorbing a liquid, when the superabsorbent polymer is applied to a product such as diapers, etc.

The present inventors have paid attention to especially the content ofthe water-soluble component that is eluted after free swelling for 1hour. Conventionally, quality of the super absorbent polymer has beendetermined by the content of the water-soluble component that is elutedafter free swelling for 16 hours, and this content denotes the totalcontent of the water-soluble component. However, the experimentalresults of the present invention are determined after free swelling for1 hour. At least some experimental results, such as the content andmolecular weight distribution of the water-soluble component afterswelling for 16 hours may not be directly comparable to the content andmolecular weight distribution of the water-soluble component afterswelling for 1 hour. For example, a SAP of the present invention mayhave comparable experimental results to other SAPs when measured afterswelling for 16 hours. However, a SAP of the present invention may nothave comparable experimental result to other SAPs when measured afterswelling for 1 hr. Further, it was found that physical propertiesgreatly affecting the quality of the SAP are the content of thewater-soluble component after free swelling for 1 hour, since mostphysical properties of the SAP, including liquid permeability, aremeasured based on 1 hr-swelling.

Accordingly, by controlling the total content and the contentdistribution according to the molecular weight of the water-solublecomponent, the super absorbent polymer of the present invention may havehigh centrifuge retention capacity and excellent liquid permeability,and the content of the water-soluble component that is eluted uponswelling may be minimized, thereby reducing discomfort.

While the super absorbent polymer is swollen, the water-solublecomponent in the polymer is eluted. At an initial stage, a water-solublecomponent having a low molecular weight is eluted, and a water-solublecomponent having a high molecular weight is eluted over time. Inparticular, upon swelling for 1 hour, a water-soluble component havingmolecular weights ranging from 100,000 to 300,000 g/mol are mostlyeluted. Therefore, it was found that the amount of the water-solublecomponent upon 1 hr-swelling can be reduced by controlling the amount ofthe water-soluble component having a molecular weight of 100,000 to300,000 g/mol which mainly influences the amount of the water-solublecomponent upon 1 hr-swelling, leading to the present invention.

In this regard, an amount of the water-soluble component base on thetotal weight of the super absorbent polymer may be measured inaccordance with EDANA WSP 270.3, in particular, a measurement methodafter free swelling for 1 hour. That is, the water-soluble component maybe included in an amount of 5% by weight or less, based on the totalweight of the super absorbent polymer, when measured after free swellingthe super absorbent polymer of the present invention with 0.9% NaClsolution for 1 hour. As used herein, “free swelling” means that thesuper absorbent polymer is allowed to swell without a restraining loadupon absorbing the saline solution.

Further, the super absorbent polymer of the present invention may have apermeability of 50 seconds or shorter, for example, about 1 to about 50seconds, preferably about 1 to about 40 seconds, or more preferablyabout 1 to about 35 seconds, which is calculated by the followingEquation 1:

Permeability (sec)=T _(S) −T ₀  [Equation 1]

wherein T_(S) (sec) is the time required for an amount of a 0.9% salinesolution to permeate a saline-absorbed super absorbent polymer under aload of 0.3 psi, wherein the saline-absorbed super absorbent polymer isprepared by swelling 0.2 g of super absorbent polymer powder with the0.9% saline solution for 30 minutes, and

T₀ (sec) is the time required for the amount of the 0.9% saline solutionto flow under the load of 0.3 psi in the absence of the saline-absorbedsuper absorbent polymer.

The permeability is an index showing how well a saline solution (0.9%NaCl aqueous solution) permeates the swollen super absorbent polymer,and evaluated by measuring the time taken for 0.9% saline solution topermeate after swelling 0.2 g of the super absorbent polymer powder for30 minutes and then applying a pressure of 0.3 psi, as described in theliterature (Buchholz, F. L. and Graham, A. T., “Modern Super absorbentPolymer Technology,” John Wiley & Sons (1998), page 161). A moredetailed description of the permeability measurement method will begiven in the following Examples.

The super absorbent polymer of the present invention exhibits a highpermeation rate for saline solution, and sufficient centrifuge retentioncapacity and absorbency under load, while having a very low content ofthe water-soluble component that is eluted upon swelling for 1 hour,thereby showing great improvements in the problem of sticky surface orskin discomfort. Therefore, the super absorbent polymer has a balancebetween conflicting physical properties of centrifuge retentioncapacity, permeation rate, and absorption rate, and thus it may beproperly used as a filler for thin or ultrathin hygiene products.

According to an embodiment of the present invention, in the superabsorbent polymer of the present invention, the water-soluble componenthas a ratio (dwt/d(log M)) of 0.9 or less over molecular weights (M)ranging from 100,000 to 300,000 g/mol when measured from an elutedsolution after free swelling the super absorbent polymer with 0.9% NaClsolution for 1 hour.

More particularly, 1 g of the super absorbent polymer of the presentinvention is put in a 250 mL-Erlenmeyer flask, and swollen in 200 mL of0.9% NaCl solution at 25° C. under shaking at 500 rpm for 1 hour. Then,the molecular weight distribution was measured by GPC. The water-solublecomponent having a ratio (dwt/d(log M)) over molecular weights (M)ranging from 100,000 to 300,000 g/mol, the ratio is about 0.9 or less,preferably about 0.86 or less, and more preferably about 0.8 or less.

When the super absorbent polymer has the content and the contentdistribution according to molecular weight of the water-solublecomponent as described above, the water-soluble component which iseluted upon swelling is minimized, thereby reducing discomfort.

The super absorbent polymer may have centrifuge retention capacity (CRC)of about 26 to about 32 g/g, and preferably about 27 to about 31 g/g,which is measured in accordance with EDANA WSP 241.2.

Further, the super absorbent polymer may have absorbency under load (0.7psi AUL) of about 20 to about 26 g/g, and preferably about 22 to about25 g/g, which is measured in accordance with EDANA WSP 242.2.

As described above, the present invention provides a super absorbentpolymer having excellent liquid permeability even when swollen without areduction in centrifuge retention capacity or absorbency under load.

The super absorbent polymer of the present invention having the abovecharacteristics may be achieved by adjusting conditions related tointernal crosslinking and surface crosslinking reactions. The totalcontent of the water-soluble component and the molecular weight of thewater-soluble component greatly differ depending on the content of aninitiator, polymerization temperature, content of a crosslinking agent,conditions for a surface crosslinking process, etc. during thepolymerization. The conventional method for controlling thewater-soluble component is a method of carrying out a post-process suchas neutralization after polymerization of the base resin, mixing with anadditive, or increasing the content of the crosslinking agent, etc.However, these methods are disadvantageous in that the overallproductivity of the super absorbent polymer is reduced, or waterabsorption property is reduced.

Meanwhile, according to the present invention, the content distributionof the water-soluble component is adjusted by controlling the conditionsfor the internal crosslinking and surface crosslinking processes withoutan additional process or injection of an additive, thereby optimizingwater absorption property of the super absorbent polymer and physicalproperties of the water-soluble component, leading to preparation of abalanced polymer.

According to an embodiment of the present invention, therefore, thesuper absorbent polymer of the present invention may be prepared by apreparation method including the steps of performing thermalpolymerization or photo-polymerization of a monomer compositioncontaining water-soluble ethylene-based unsaturated monomers and apolymerization initiator so as to prepare a hydrogel polymer; drying thehydrogel polymer; pulverizing the dried polymer; and performing asurface crosslinking reaction by heating a mixture of the pulverizedpolymer with a surface crosslinking solution containing a surfacecrosslinking agent and water at 180 to 200° C.

In the preparation method of the super absorbent polymer of the presentinvention, the monomer composition which is a raw material of the superabsorbent polymer includes the water-soluble ethylene-based unsaturatedmonomers and the polymerization initiator.

The water-soluble ethylene-based unsaturated monomer may be any monomerthat is typically used in the preparation of the super absorbent polymerwithout limitation. Herein, one or more monomers selected from the groupconsisting of an anionic monomer and salts thereof, a nonionichydrophilic monomer and an amino group-containing unsaturated monomer,and a quaternary compound thereof may be used.

Specifically, one or more selected from the group consisting of ananionic monomer such as acrylic acid, methacrylic acid, maleicanhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethane sulfonic acid, 2-acryloylpropanesulfonic acid, 2-methacryloylpropane sulfonic acid,2-acrylamide-2-methyl propane sulfonic acid or 2-methacrylamide-2-methylpropane sulfonic acid, and salts thereof; a nonionic hydrophilic monomersuch as acrylamide, methacrylamide, N-substituted acrylate,N-substituted methacrylate, 2-hydroxyethylacrylate,2-hydroxyethylmethacrylate, 2-hydroxypropylacrylate,2-hydroxypropylmethacrylate, methoxy polyethylene glycol acrylate,methoxy polyethylene glycol methacrylate, polyethylene glycol acrylateor polyethylene glycol methacrylate; and an amino group-containingunsaturated monomer such as (N,N)-dimethylaminoethylacrylate,(N,N)-dimethylaminoethylmethacrylate,(N,N)-dimethylaminopropylacrylamide or(N,N)-dimethylaminopropylmethacrylamide, and a quaternary compoundthereof may be used.

More preferably, acrylic acid or salts thereof, for example, acrylicacid or alkali metal salts such as sodium salts thereof may be used, andit is possible to prepare a super absorbent polymer having superiorphysical properties by using these monomers. When the alkali metal saltof acrylic acid is used as the monomer, acrylic acid may be used afterneutralized with a basic compound such as caustic soda (NaOH). The basiccompound may be used in an amount of about 20 to 60 parts by weight,preferably about 30 to 50 parts by weight, based on 100 parts by weightof the acrylic acid.

The concentration of the water-soluble ethylene-based unsaturatedmonomer may be about 20 to about 60% by weight, preferably about 40 toabout 50% by weight, based on the monomer composition. The monomercomposition may include a solvent, and the concentration may be properlycontrolled, considering polymerization time and reaction conditions.However, if the monomer concentration is too low, the yield of the superabsorbent polymer may become low and an economic problem may occur. Onthe contrary, if the concentration is too high, there is a processproblem that a part of the monomers is precipitated, or pulverizationefficiency is lowered upon pulverization of the polymerized hydrogelpolymer, and the physical properties of the super absorbent polymer maybe reduced.

In the preparation method of the super absorbent polymer of the presentinvention, the polymerization initiator used in polymerization is notparticularly limited, as long as it is generally used in the preparationof the super absorbent polymer.

Specifically, the polymerization initiator may be a thermalpolymerization initiator or a photo-polymerization initiator by UVirradiation, depending on a polymerization method. However, even thoughthe photo-polymerization is performed, a certain amount of heat isgenerated by UV irradiation or the like and is also generated withexothermic polymerization reaction. Therefore, the thermalpolymerization initiator may be further included.

As the photo-polymerization initiator, a compound capable of formingradicals by a light such as UV may be used without limitations in theconstitution.

For example, one or more selected from the group consisting of benzoinether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate,benzyl dimethyl ketal, acyl phosphine, and α-aminoketone may be used asthe photo-polymerization initiator. Meanwhile, as the specific exampleof acyl phosphine, commercialized Lucirin TPO, namely,2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide, may be used. Morevarious photo-polymerization initiators are well disclosed in “UVCoatings: Basics, Recent Developments and New Application (Elsevier,2007)” written by Reinhold Schwalm, p115, however, they are not limitedto the above described examples.

The concentration of the photo-polymerization initiator may be about0.01 to about 1.0% by weight, based on the monomer composition. If theconcentration of the photo-polymerization initiator is too low, apolymerization rate may become low. If the concentration of thephoto-polymerization initiator is too high, the molecular weight of thesuper absorbent polymer may be decreased and its physical properties maybe not uniform.

Further, one or more selected from the group consisting ofpersulfate-based initiators, azo-based initiators, hydrogen peroxide andascorbic acid may be used as the thermal polymerization initiator.Specific examples of the persulfate-based initiators may include sodiumpersulfate (Na₂S₂O₈), potassium persulfate (K₂S₂O₈), ammonium persulfate((NH₄)₂S₂O₈) or the like. Examples of the azo-based initiators mayinclude 2,2-azobis(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutylonitril,2,2-azobis(2-[2-imidazolin-2-yl]propane)dihydrochloride,4,4-azobis-(4-cyanovaleric acid) or the like. More various thermalpolymerization initiators are well-disclosed in ‘Principle ofPolymerization (Wiley, 1981)’ written by Odian, p203, however, they arenot limited to the above described examples.

The concentration of the thermal polymerization initiator may be about0.001 to about 0.5% by weight, based on the monomer composition. If theconcentration of the thermal polymerization initiator is too low,additional thermal polymerization hardly occurs, and thus the additioneffect of the thermal polymerization initiator may not be sufficientlyobtained. If the concentration of the thermal polymerization initiatoris too high, the molecular weight of the super absorbent polymer may bedecreased and its physical properties may be not uniform.

According to an embodiment of the present invention, the monomercomposition may further include an internal crosslinking agent. Suchinternal crosslinking agent may be a crosslinking agent which has one ormore of the functional group capable of reacting with a water-solublesubstituent of the water-soluble ethylene-based unsaturated monomer andhas one or more ethylenic unsaturated groups; or a crosslinking agentwhich has two or more of the functional group capable of reacting with awater-soluble substituent of the monomer and/or a water-solublesubstituent formed by hydrolysis of the monomer.

Specific example of the internal crosslinking agent may include acrosslinking agent having two or more (meth)acrylate groups binding topolyol, for example, di(meth)acrylate of polyol having 2 to 20 carbonatoms, poly(meth)acrylate of polyol having 2 to 20 carbon atoms, etc.More specifically, one or more selected from the group consisting ofpolyethylene glycol diacrylate, hexanediol diacrylate, glycerindiacrylate, glycerin triacrylate, and trimethylol triacrylate may beused.

Such internal crosslinking agent is included at a concentration of about0.01 to about 0.5% by weight based on the monomer composition so as tocrosslink the polymerized polymer.

The super absorbent polymer of the present invention has a low contentof the water-soluble component eluted upon swelling and highpermeability while having proper centrifuge retention capacity andabsorbency under load by controlling the kind and content of theinternal crosslinking agent during polymerization of the water-solubleethylene-based unsaturated monomers to prepare a high-strength baseresin having a high crosslinking density and then by optimizingconditions for the after-mentioned surface crosslinking process of thehigh-strength base resin. Therefore, it may be very favorably applied toproducts, considering the current trend of slimness of the absorbentthickness.

In the preparation method of the present invention, the monomercomposition of the super absorbent polymer may further include anadditive such as a thickener, a plasticizer, a preservation stabilizer,an antioxidant, etc., if necessary.

The components of a monomer composition, such as the above describedwater-soluble ethylene-based unsaturated monomer, photo-polymerizationinitiator, thermal polymerization initiator, internal crosslinking agentand additive, can include a solvent.

In this regard, a solvent capable of dissolving the above ingredientsmay be used as the solvent without limitations in the constitution, andfor example, one or more selected from water, ethanol, ethylene glycol,diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol,ethylene glycol monobutyl ether, propylene glycol monomethyl ether,propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone,methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycolmonomethyl ether, diethylene glycol ethylether, toluene, xylene,butyrolactone, carbitol, methyl cellosolve acetate andN,N-dimethylacetamide may be used in combination.

The solvent may be included in any suitable amount based on the totalweight of the monomer composition. In one embodiment, the solvent may beincluded in an amount ranging from about 20 to about 75% by weight,preferably about 40 to about 70% by weight, based on the total weight ofthe monomer composition.

Meanwhile, the method for forming a hydrogel polymer by thermalpolymerization or photo-polymerization of the monomer composition is notparticularly limited in the constitution, as long as it is a methodtypically used.

Specifically, the polymerization method is largely classified into thethermal polymerization and the photo-polymerization according to thepolymerization energy source, and the thermal polymerization may becarried out in a reactor like a kneader equipped with agitating spindlesand the photo-polymerization may be carried out in a reactor equippedwith a movable conveyor belt. The above described polymerization methodis an example only, and the present invention is not limited thereto.

For example, as described above, thermal polymerization is performed byproviding hot air to a reactor like a kneader equipped with theagitating spindles or by heating the reactor so as to obtain thehydrogel polymer. At this time, the hydrogel polymer may have the sizeof centimeters or millimeters when it is discharged from the outlet ofthe reactor, according to the type of agitating spindles equipped in thereactor. Specifically, the hydrogel polymer may be obtained in variousforms according to the concentration of the monomer composition fedthereto, the feeding speed or the like, and the hydrogel polymer havinga weight average particle size of 2 to 50 mm may be generally obtained.

Further, as described above, when the photo-polymerization is carriedout in a reactor equipped with a movable conveyor belt, the hydrogelpolymer typically obtained may be the hydrogel polymer in a sheet-typehaving a width of the belt. In this regard, the thickness of the polymersheet may vary according to the concentration of the monomer compositionfed thereto and the feeding speed. Preferably, the monomer compositionis fed so that the polymer sheet has a thickness of about 0.5 to about 5cm. If the monomer composition is fed so that the thickness of thesheet-type polymer becomes too thin, the production efficiency becomeslow, which is not preferred. If the thickness of the sheet-type polymerexceeds 5 cm, the polymerization reaction may not uniformly occurthroughout the polymer due to the excessively high thickness.

In this regard, the hydrogel polymer thus obtained by the method mayhave typically a water content of about 40 to about 80% by weight.Meanwhile, the term “water content”, as used herein, means a watercontent in the total weight of the hydrogel polymer, which is obtainedby subtracting the weight of the dry polymer from the weight of thehydrogel polymer. Specifically, it is defined as a value calculated bymeasuring the weight loss according to evaporation of water in thepolymer during the drying process of increasing the temperature of thepolymer with infrared heating. In this regard, the water content ismeasured under the drying conditions which are determined as follows;the temperature is increased from room temperature to about 180° C. andthen the temperature is maintained at 180° C., and the total drying timeis determined as 20 minutes, including 5 minutes for the temperaturerising step.

Next, the step of drying the hydrogel polymer thus obtained isperformed.

If necessary, a coarsely pulverizing step may be performed before thedrying step, in order to increase the efficiency of the drying step.

In this regard, a pulverizing device applicable may include, but theconstitution is not limited, any one selected from the group consistingof a vertical pulverizer, a turbo cutter, a turbo grinder, a rotarycutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, achopper, and a disc cutter, but is not limited thereto.

In this regard, the pulverizing step may be performed so that thehydrogel polymer has a particle size of about 2 to about 10 mm.

To pulverize the polymer to have a particle size of less than 2 mm istechnically not easy due to a high water content of the hydrogelpolymer, and agglomeration may occur between the pulverized particles.If the polymer is pulverized to have a particle size of more than 10 mm,the effect of increasing the efficiency in the succeeding drying stepmay be insignificant.

The hydrogel polymer pulverized as above or immediately afterpolymerization without the pulverizing step is subjected to a dryingprocess. In this regard, the drying temperature of the drying step maybe about 150 to about 250° C. When the drying temperature is lower than150° C., there is a concern that the drying time becomes excessivelylong or the physical properties of the super absorbent polymer finallyformed may be deteriorated, and when the drying temperature is higherthan 250° C., only the surface of the polymer is dried, and thus thereis a concern that fine powder may be generated during the subsequentpulverization process and the physical properties of the super absorbentpolymer finally formed may be deteriorated. Therefore, the dryingprocess may be preferably performed at a temperature of about 150 toabout 200° C., and more preferably about 160 to about 180° C.

Meanwhile, the drying process may be carried out for about 20 to about90 minutes, considering the process efficiency, but is not limitedthereto.

Furthermore, any known drying method may be selected and used in thedrying step without limitation in the constitution if it can begenerally used for drying the hydrogel polymer. Specifically, the dryingstep may be carried out by a method of supplying hot air, irradiatinginfrared rays, irradiating microwaves, irradiating ultraviolet rays orthe like. When the drying step as above is finished, the water contentof the polymer may be about 0.1 to about 10% by weight.

Next, the dried polymer obtained from the drying step is subjected to apulverization step.

The polymer powder obtained from the pulverization step may have aparticle size of about 150 to about 850 μm. Specific example of amilling device that can be used to achieve the above particle size mayinclude a pin mill, a hammer mill, a screw mill, a roll mill, a discmill, a jog mill or the like, but the present invention is not limitedthereto.

The physical properties of the super absorbent polymer powder finallymanufactured after the pulverization step may be properly controlledthrough a sorting step according to the particle size of the polymerpowder obtained from the pulverization. Only a polymer having a particlesize of about 150 to about 850 μm is preferably sorted and thenselectively applied to the surface crosslinking reaction and finally, itis commercialized.

Next, a surface crosslinking reaction is performed by mixing thepulverized polymer with a surface crosslinking solution containing asurface crosslinking agent and water.

The surface crosslinking is a step of increasing the crosslinkingdensity in the vicinity of the surface of the super absorbent polymerparticle with regard to the internal crosslinking density of particles.In general, the surface crosslinking agent is applied to the surface ofthe super absorbent polymer particle. Therefore, this reaction occurs onthe surface of the super absorbent polymer particle, which improvescrosslinking on the surface of the particle without substantiallyaffecting the interior of the particle. Thus, the surface-crosslinkedsuper absorbent polymer particles have a higher level of crosslinking inthe vicinity of the surface than in the interior.

In this regard, a compound capable of reacting with the functionalgroups of the polymer may be used as the surface crosslinking agentwithout limitations in the constitution.

To improve the properties of the produced super absorbent polymer, oneor more selected from the group consisting of a polyhydric alcoholcompound; an epoxy compound; a polyamine compound; a haloepoxy compound;a condensation product of the haloepoxy compound; an oxazoline compound;a mono-, di-, or polyoxazolidinone compound; a cyclic urea compound; apolyvalent metal salt; and an alkylene carbonate compound may bepreferably used as the surface crosslinking agent.

Specific examples of the polyhydric alcohol compound may be one or moreselected from the group consisting of a mono-, di-, tri-, tetra-, orpolyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropyleneglycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol,1,5-pentanediol, 1,6-hexanediol, and 1,2-cyclohexane dimethanol.

Further, the epoxy compound may be ethylene glycol diglycidyl ether,glycidol, etc. and the polyamine compound may be one or more selectedfrom the group consisting of ethylene diamine, diethylene triamine,triethylene tetraamine, tetraethylene pentamine, pentaethylene hexamine,polyethylene imine, and polyamide polyamine.

Further, the haloepoxy compound may be epichlorohydrin, epibromohydrin,or α-methylephichlorohydrin. Meanwhile, the mono-, di-, orpolyoxazolidinone compound may be, for example, 2-oxazolidinone.

Further, the alkylene carbonate compound may be ethylene carbonate, etc.These may be used singly or in combination. On the other hand, toincrease the efficiency of the surface crosslinking process, one or moreof polyhydric alcohols are preferably included in these surfacecrosslinking agents. More preferably, polyhydric alcohol compoundshaving 2 to 10 carbon atoms may be used.

The amount of the surface crosslinking agent added may be suitablycontrolled according to the kind of the surface crosslinking agent addedor the reaction conditions, and it may be typically used in an amount ofabout 0.001 to about 5 parts by weight, preferably about 0.01 to about 3parts by weight, and more preferably about 0.05 to about 2 parts byweight, based on 100 parts by weight of the polymer.

When the amount of the surface crosslinking agent used is too small, thesurface crosslinking reaction hardly occurs, and when the surfacecross-linking agent is used in an amount of more than 5 parts by weightbased on 100 parts by weight of the polymer, the absorptivity and thephysical properties may be decreased due to excessive surfacecrosslinking reaction.

The surface crosslinking agent may be added in the form of a surfacecrosslinking solution by mixing it with additional water. When water isadded, there is advantageous in that the surface crosslinking agent canbe evenly dispersed in the polymer. In this regard, the amount of wateradded thereto may be preferably about 1 to about 10 parts by weightbased on 100 parts by weight of the polymer for the purpose of inducinguniform dispersion of the surface cross-linking agent, preventingagglomeration of the polymer powder, and optimizing the surfacepenetrating depth of the cross-linking agent at the same time.

Further, the surface crosslinking solution may further include asubstance such as a metal salt, silica, etc.

With regard to the method of adding the surface crosslinking solution tothe polymer, there is no limitation in the constitution. A method ofadding and mixing the surface crosslinking solution and the polymerpowder in a reactor, a method of spraying the surface crosslinkingsolution onto the polymer powder, or a method of continuously feedingthe polymer and the surface crosslinking solution to a mixer which iscontinuously operated may be used.

As described above, the total content of the water-soluble component andthe molecular weight of the water-soluble component greatly differdepending on the content of the initiator, polymerization temperature,content of the crosslinking agent, conditions for the surfacecrosslinking process, etc. during the polymerization. According to thepresent invention, a super absorbent polymer having the above describedwater-soluble component distribution may be prepared by controlling thetemperature within a specific range during the surface crosslinkingprocess.

According to the preparation method of the super absorbent polymer ofthe present invention, the surface crosslinking reaction is allowed tooccur by heating the surface crosslinking solution-added polymerparticles at about 180 to about 200° C., preferably at about 180 toabout 190° C. When the reaction temperature is within the above range, asuper absorbent polymer having the above described properties ofwater-soluble component may be obtained.

Further, the surface crosslinking reaction and drying may occur at thesame time by heating for a crosslinking reaction time of about 15 toabout 90 minutes, preferably about 20 to about 80 minutes, and morepreferably about 40 to about 80 minutes. If the crosslinking reactiontime is too short, sufficient crosslinking reaction may not occur, andif the crosslinking reaction time is too long, the polymer particles aredamaged due to excessive surface crosslinking reaction, leading todeterioration in the physical properties.

A means for raising the temperature for surface crosslinking reaction isnot particularly limited. Heating may be performed by providing aheating medium or by directly providing a heat source. In this regard,the type of the heating medium applicable may be a hot fluid such assteam, hot air, hot oil, or the like. However, the present invention isnot limited thereto. The temperature of the heating medium provided maybe properly controlled, considering the means of the heating medium, theheating rate, and the target temperature. Meanwhile, as the heat sourceprovided directly, an electric heater or a gas heater may be used, butthe present invention is not limited to these examples.

With regard to the super absorbent polymer of the present inventionwhich is obtained by the above preparation method, the content of theeluted water-soluble component is 5% by weight or less based on thetotal weight of the super absorbent polymer, when measured after adding1 g of the super absorbent polymer in a 250 mL-Erlenmeyer flask, andthen free swelling it in 200 mL of 0.9% NaCl solution at 25° C. undershaking at 500 rpm for 1 hour.

Further, the super absorbent polymer of the present invention which isobtained by the above preparation method may have the permeability of 50seconds or shorter, for example, about 1 to about 50 seconds, preferablyabout 1 to about 40 seconds, and more preferably about 1 to about 35seconds.

Further, the water-soluble component has a ratio (dwt/d(log M)) of 0.9or less, preferably 0.86 or less, and more preferably about 0.8 or lessover molecular weights (M) ranging from 100,000 to 300,000 g/mol whenmeasured by GPC.

Further, the super absorbent polymer of the present invention which isobtained by the above preparation method has centrifuge retentioncapacity of about 26 to about 32 g/g, and preferably about 27 to about31 g/g, which is measured in accordance with EDANA WSP 241.2, andabsorbency under load (0.7 psi AUL) of about 20 to about 26 g/g, andpreferably about 22 to about 25 g/g, which is measured in accordancewith EDANA WSP 242.2, thereby showing excellent centrifuge retentioncapacity and absorbency under load.

As such, the super absorbent polymer of the present invention exhibitsexcellent liquid permeability even when swollen without a reduction inphysical properties such as centrifuge retention capacity, absorbencyunder load, etc.

The present invention will be described in more detail with reference tothe following Examples. However, the following Examples are forillustrative purposes only, and the present invention is not intended tobe limited by these Examples.

EXAMPLE Example 1

A monomer composition in a form of aqueous solution including 100 partsby weight of acrylic acid (water-soluble ethylene-based unsaturatedmonomer), 30 parts by weight of NaOH, 0.03 parts by weight of potassiumigacure 651 (photo-polymerization initiator), 0.5 parts by weight ofpolyethylene glycol diacrylate (crosslinking agent), 0.1 parts by weightof 1,6-hexanediol diacrylate (crosslinking agent), and 110 parts byweight of water as a solvent was prepared. The contents of componentsother than the acrylic acid were defined as parts by weight based on 100parts by weight of the acrylic acid. The monomer composition was fed ata feed rate of 500 mL/min to 2,000 mL/min on a conveyor belt having awidth of 10 cm and a length of 2 m and rotating at a speed of 50 cm/min.

While the monomer composition was fed, UV having an intensity of 10mW/cm² was irradiated, and polymerization was allowed for 60 seconds.After polymerization, the resultant was cut with a meat chopper, andthen dried using a convection oven at 160° C. for 5 hours, followed bypulverizing and sorting processes. Thus, a polymer having a particlesize ranging from 150 to 850 μm was obtained.

Based on 100 parts by weight of the polymer, 0.4 parts by weight ofethylene carbonate, 5 parts by weight of methanol, and 4 parts by weightof water were mixed with each other, and then surface treatment reactionwas allowed in a convection oven at 190° C. for 40 minutes so as toprepare a super absorbent polymer.

Example 2

A super absorbent polymer was prepared in the same manner as in Example1, except that based on 100 parts by weight of the polymer of Example 1,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 200°C. for 40 minutes.

Example 3

A super absorbent polymer was prepared in the same manner as in Example1, except that based on 100 parts by weight of the polymer of Example 1,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 180°C. for 80 minutes.

Example 4

A monomer composition in a form of aqueous solution including 100 partsby weight of acrylic acid (water-soluble ethylene-based unsaturatedmonomer), 30 parts by weight of NaOH, 0.03 parts by weight of potassiumigacure 651 (photo-polymerization initiator), 0.5 parts by weight ofpolyethylene glycol diacrylate (crosslinking agent), and 110 parts byweight of water as a solvent was prepared. The contents of componentsother than the acrylic acid were defined as parts by weight based on 100parts by weight of the acrylic acid. The monomer composition was fed ata feed rate of 500 mL/min to 2,000 mL/min on a conveyor belt having awidth of 10 cm and a length of 2 m and rotating at a speed of 50 cm/min.

While the monomer composition was fed, UV having an intensity of 10mW/cm² was irradiated, and polymerization was allowed for 60 seconds.After polymerization, the resultant was cut with a meat chopper, andthen dried using a convection oven at 160° C. for 5 hours, followed bypulverizing and sorting processes. Thus, a polymer having a particlesize ranging from 150 to 850 μm was obtained.

Based on 100 parts by weight of the polymer, 0.4 parts by weight ofethylene carbonate, 5 parts by weight of methanol, and 4 parts by weightof water were mixed with each other, and then surface treatment reactionwas allowed in a convection oven at 190° C. for 40 minutes so as toprepare a super absorbent polymer.

Example 5

A super absorbent polymer was prepared in the same manner as in Example4, except that based on 100 parts by weight of the polymer of Example 4,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 200°C. for 40 minutes.

Example 6

A super absorbent polymer was prepared in the same manner as in Example4, except that based on 100 parts by weight of the polymer of Example 4,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 180°C. for 80 minutes.

Comparative Example 1

A monomer composition in a form of aqueous solution including 100 partsby weight of acrylic acid (water-soluble ethylene-based unsaturatedmonomer), 30 parts by weight of NaOH, 0.03 parts by weight of potassiumigacure 651 (photo-polymerization initiator), 0.3 parts by weight ofpolyethylene glycol diacrylate (crosslinking agent), and 110 parts byweight of water as a solvent was prepared. The contents of componentsother than the acrylic acid were defined as parts by weight based on 100parts by weight of the acrylic acid. The monomer composition was fed ata feed rate of 500 mL/min to 2,000 mL/min on a conveyor belt having awidth of 10 cm and a length of 2 m and rotating at a speed of 50 cm/min.

While the monomer composition was fed, UV having an intensity of 10mW/cm² was irradiated, and polymerization was allowed for 60 seconds.After polymerization, the resultant was cut with a meat chopper, andthen dried using a convection oven at 160° C. for 5 hours, followed bypulverizing and sorting processes. Thus, a polymer having a particlesize ranging from 150 to 850 μm was obtained.

Based on 100 parts by weight of the polymer, 0.4 parts by weight ofethylene carbonate, 5 parts by weight of methanol, and 4 parts by weightof water were mixed with each other, and then surface treatment reactionwas allowed in a convection oven at 210° C. for 20 minutes so as toprepare a super absorbent polymer.

Comparative Example 2

A super absorbent polymer was prepared in the same manner as inComparative Example 1, except that based on 100 parts by weight of thepolymer of Comparative Example 1, 0.4 parts by weight of ethylenecarbonate, 5 parts by weight of methanol, and 4 parts by weight of waterwere mixed with each other, and then surface treatment reaction wasallowed in a convection oven at 175° C. for 90 minutes.

Comparative Example 3

A super absorbent polymer was prepared in the same manner as in Example1, except that based on 100 parts by weight of the polymer of Example 1,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 210°C. for 20 minutes.

Comparative Example 4

A super absorbent polymer was prepared in the same manner as in Example1, except that based on 100 parts by weight of the polymer of Example 1,0.4 parts by weight of ethylene carbonate, 5 parts by weight ofmethanol, and 4 parts by weight of water were mixed with each other, andthen surface treatment reaction was allowed in a convection oven at 175°C. for 90 minutes.

Comparative Example 5

A super absorbent polymer was prepared in the same manner as inComparative Example 1, except that based on 100 parts by weight of thepolymer of Comparative Example 1, 0.4 parts by weight of ethylenecarbonate, 5 parts by weight of methanol, and 4 parts by weight of waterwere mixed with each other, and then surface treatment reaction wasallowed in a convection oven at 190° C. for 40 minutes.

Comparative Example 6

A super absorbent polymer was prepared in the same manner as inComparative Example 1, except that based on 100 parts by weight of thepolymer of Comparative Example 1, 0.4 parts by weight of ethylenecarbonate, 5 parts by weight of methanol, and 4 parts by weight of waterwere mixed with each other, and then surface treatment reaction wasallowed in a convection oven at 200° C. for 40 minutes.

Comparative Example 7

A super absorbent polymer was prepared in the same manner as inComparative Example 1, except that based on 100 parts by weight of thepolymer of Comparative Example 1, 0.4 parts by weight of ethylenecarbonate, 5 parts by weight of methanol, and 4 parts by weight of waterwere mixed with each other, and then surface treatment reaction wasallowed in a convection oven at 180° C. for 80 minutes.

Experimental Example

Analysis of Water-Soluble Component

Each 1.0 g of the samples having a particle size of 150 to 850 μm wastaken from the super absorbent polymers prepared in Examples andComparative Examples and put in a 250 mL-Erlenmeyer flask, andfree-swollen in 200 mL of 0.9% NaCl solution under shaking at 250 rpmfor 1 hour. The aqueous solution was passed through a filter paper, andthe filtrate was primarily titrated to pH 10 with 0.1 N caustic soda,and then back-titrated to pH 2.7 with 0.1 N hydrogen chloride solution.The content (% by weight) of water-soluble component in the superabsorbent polymer was calculated from the obtained titration inaccordance with EDANA WSP 270.3.

Further, 100 μL of the aqueous solution that was passed through a filterpaper was injected into GPC instrument to measure the ratio of dwt/d(logM). GPC was performed on an instrument manufactured by Wyatt DAWN EOS,Wyatt Optilab DSP, Waters, or Wyatt, using Ultrahydrogel Linear X2 as acolumn, 0.1 M NaNO₃/0.02 M phosphate buffer as a solvent under theconditions of a flow rate of 0.8 mL/min and a temperature of 60° C., andpolyacrylic acid was used as a standard.

The analysis results were obtained as dwt/d(log M) for log M (M is amolecular weight of eluted water-soluble component), and thus log Mvalues were converted to M values, thereby obtaining dwt/d(log M) foreach molecular weight of 100,000, 200,000, or 300,000.

The measurement results of the water-soluble components of Examples andComparative Examples are shown in the following Table 1.

TABLE 1 Content of water-soluble component (free welling for 1 h; % bydwt/d(logM) dwt/d(logM) dwt/d(logM) weight) (M: 100,000) (M: 200,000)(M: 300,000) Example 1 2.6 0.662 0.826 0.784 Example 2 3.1 0.625 0.8140.794 Example 3 2.5 0.682 0.820 0.751 Example 4 3.8 0.648 0.844 0.801Example 5 4.2 0.633 0.854 0.813 Example 6 3.9 0.694 0.832 0.793Comparative 6.6 0.708 0.910 0.803 Example 1 Comparative 7.8 0.661 0.8920.815 Example 2 Comparative 5.8 0.695 0.921 0.713 Example 3 Comparative6.3 0.751 0.884 0.821 Example 4 Comparative 3.8 0.623 0.859 0.792Example 5 Comparative 4.3 0.613 0.850 0.819 Example 6 Comparative 3.50.597 0.783 0.745 Example 7

Meanwhile, for comparison with the results of the content ofwater-soluble component after free swelling for 1 hour, the content ofwater-soluble component after free swelling for 16 hours is measured forthe samples of the Examples 1 to 3 and the Comparative Example 3. Thecontent of water-soluble component after free swelling for 16 hours ismeasured according to the similar method to that after free swelling for1 hour, except that Each of the samples was free-swollen in 200 mL of0.9% NaCl solution under shaking at 250 rpm for 16 hour. The measurementresults thereof are shown in the following Table 2.

TABLE 2 Content of water-soluble component (free welling for 16 h; % byweight) Example 1 14.8 Example 2 15.1 Example 3 14.3 Comparative 14.0Example 3

Referring to the Table 1, the Examples show the different content andmolecular weight distribution of the water-soluble component after 1hour from the Comparative Examples. However, the Examples 1 to 3 and theComparative Example 3 do not show the substantial difference in thecontent of the water-soluble component after 16 hours.

Measurement of Permeability, Centrifuge Retention Capacity andAbsorbency Under Load

1) Permeability

Permeability was measured using a 0.9% saline solution under a load of0.3 psi, as described in the literature (Buchholz, F. L. and Graham, A.T., “Modern Superabsorbent Polymer Technology,” John Wiley & Sons(1998), page 161).

In more detail, 0.2 g of particles having a particle size of 300 to 600μm were taken from the super absorbent polymers (hereinafter, referredto as a sample) prepared in Examples and Comparative Examples, and addedto a cylinder (Φ20 mm), wherein the cylinder has a stopcock on one end,an upper limit mark and a lower limit mark thereon. The upper limit markon the cylinder is indicated at the position of which 40 ml of (saline)solution is filled into the cylinder, and the lower limit mark on thecylinder is indicated at the position of which 20 ml of (saline)solution is filled into the cylinder.

50 g of 0.9% saline solution was added to the cylinder with the stopcockin a closed position, and left for 30 minutes. Then, if necessary,additional saline solution is added to the cylinder to bring the levelof saline solution to the upper limit mark on the cylinder. Then, thecylinder including the now saline-absorbed swollen super absorbentpolymers is pressurized with a load of 0.3 pounds per square inch (psi),and left for 1 minute. Thereafter, the stopcock at the bottom of thecylinder was open to measure the time taken for the 0.9% saline solutionto pass from the upper limit mark to the lower limit mark on thecylinder. All measurements were carried out at a temperature of 24±1° C.and relative humidity of 50±10%.

The time taken to pass from the upper limit mark to the lower limit markwas measured for respective samples (Ts) and also measured in theabsence of the super absorbent polymers (T₀), and permeability wascalculated by the following Equation 1:

Permeability (sec)=T _(S) −T ₀  [Equation 1]

2) Centrifuge Retention Capacity

Centrifuge retention capacity was measured in accordance with EDANA WSP241.2. 0.2 g of the sample sorted through 30-50 mesh (particle sizeranging from 300 μm to 600 μm) was put in a tea bag, and swollen in the0.9% saline solution for 30 minutes. Then, water was removed therefromby centrifugation at 250 G for 3 minutes, followed by weighing.Centrifuge retention capacity was determined by measuring the amount ofwater retained in the super absorbent polymer.

3) Absorbency Under Load (0.7 Psi AUL)

Absorbency under load (0.7 psi AUL) was measured in accordance withEDANA WSP 242.2. In detail, after uniformly distributing 0.9 g of thesample of 850 to 150 μm in a cylinder regulated in the EDANA method andthen pressing the sample with a pressure of 0.7 psi by using a pistonand a weight, the absorbency under load was calculated as the amount ofthe 0.9% saline solution that was absorbed for 1 hour.

Centrifuge retention capacity, absorbency under load and permeability ofExamples and Comparative Example thus measured by the above methods areshown in the following Table 2.

TABLE 2 Centrifuge retention Absorbency under load Permeability capacity(g/g) (0.7 psi AUL; g/g) (sec) Example 1 28.8 23.9 21 Example 2 27.622.8 14 Example 3 28.5 22.9 33 Example 4 30.7 23.5 41 Example 5 30.423.8 41 Example 6 31.2 22.9 48 Comparative 34.5 24.9 363 Example 1Comparative 38.3 11.2 948 Example 2 Comparative 27.2 22.5 71 Example 3Comparative 31.0 13.4 851 Example 4 Comparative 34.8 25.5 153 Example 5Comparative 33.8 23.2 123 Example 6 Comparative 34.7 23.8 161 Example 7

Referring to Tables 1 and 2, the amount of the water-soluble componentthat is eluted after swelling for 1 hour is 5% by weight based on thetotal weight of the super absorbent polymer and permeability is 50seconds or shorter, and therefore, the super absorbent polymer of thepresent invention has excellent liquid permeability even when swollenwhile maintaining high absorption property without a reduction incentrifuge retention capacity or absorbency under load. Accordingly, itis expected to provide the super absorbent polymer which hasimprovements in both conflicting properties of absorbency and liquidpermeability.

What is claimed is:
 1. A super absorbent polymer, comprising: surfacecrosslinked polymer particles prepared by surface crosslinking particlesof a base resin, wherein the base resin is polymerized from a monomercomposition including water-soluble ethylene-based unsaturated monomers;and a water-soluble component, wherein the water-soluble component ispresent in an amount of 3.1% by weight or less based on the total weightof the super absorbent polymer, when measured after free swelling thesuper absorbent polymer in a 0.9% NaCl solution for 1 hour, and whereinthe super absorbent polymer has a permeability calculated by thefollowing Equation 1 of about 50 seconds or shorter:Permeability (sec)=TS−T0  [Equation 1] wherein TS (sec) is the timerequired for an amount of a 0.9% saline solution to permeate asaline-absorbed super absorbent polymer under a load of 0.3 psi, whereinthe saline-absorbed super absorbent polymer is prepared by swelling 0.2g of super absorbent polymer powder with the 0.9% saline solution for 30minutes, and T0 (sec) is the time required for the amount of the 0.9%saline solution to flow under the load of 0.3 psi in the absence of thesaline-absorbed super absorbent polymer.
 2. The super absorbent polymerof claim 1, wherein the water-soluble component having a ratio(dwt/d(log M)) of 0.9 or less over molecular weights (M) ranging from100,000 to 300,000 g/mol, when measured from an eluted solution afterfree swelling 1 g of the super absorbent polymer in 200 ml of a 0.9%NaCl solution for 1 hour.
 3. The super absorbent polymer of claim 1,wherein a centrifuge retention capacity (CRC) ranges from about 26 toabout 32 g/g.
 4. The super absorbent polymer of claim 1, wherein anabsorbency under load (0.7 psi AUL) ranges from about 20 to about 26g/g.
 5. The super absorbent polymer of claim 1, wherein the base resinis polymerized in the presence of an internal cross linking agent, whichcrosslinks polymers present in the interior of the polymer particles. 6.The super absorbent polymer of claim 5, wherein the internal crosslinking agent is present in an amount ranging from 0.1 to 0.5 wt % basedon the monomer composition.
 7. The super absorbent polymer of claim 1,wherein surface crosslinked polymer particles are prepared by surfacecrosslinking particles of a base resin using a surface crosslinkingsolution including a surface crosslinking agent at a temperature rangingfrom 180 to 200° C.
 8. A hygiene product including the super absorbentpolymer of claim
 1. 9. A disposable diaper including the super absorbentpolymer of claim 1.